This invention relates to an airborne flare control system and more particularly to an airborne flare control system for providing a near asymptotic approach to touchdown.
The need for a highly reliable and safe automatic or semi-automatic landing system for aircraft has been recognized. It has also been recognized that any such systems should be fully operable under adverse weather conditions with reliability. Such systems must automatically or semi-automatically land an aircraft safely in fog or rain, at night, with heavy payloads and produce a smooth near asymptotic landing under any such conditions.
In order to meet the requirements for reliability and to provide for smooth landings under adverse conditions, an automatic or semi-automatic landing system must contain control equipment which is insensitive to outside disturbances normally encountered in landings. Such systems must also be reliable, durable, reasonably easy to install and to operate with minimum maintenance.
In the past, various control systems have been used to control aircraft in flight. For example, glide slope systems have been used to guide an aircraft down to a flare altitude of about 50 feet. However, at flare altitude, other means of control are provided to alter the aircraft""s flight path and achieve a touchdown at a much reduced rate of descent.
One system for automatically landing an aircraft is disclosed in U.S. Pat. No. 3,031,662 of Bond. In that system, a flare path is provided which accurately controls the rate of descent of the aircraft in proportion to the altitude. If the rate of descent of an aircraft is controlled in proportion to altitude, an exponential flare may be obtained which provides the necessary flare path to meet the design requirements.
In one embodiment of the Bond system, the altitude and altitude rate signals are obtained from a radio altimeter and the acceleration signal is obtained from a vertical accelerometer. The three signals are then combined to provide an indication to an automatic control system of the relation of the actual landing path described by the aircraft to the theoretical curve described by an altitude equation.
Notwithstanding past developments and the need for reliable automatic landing systems, such systems have not enjoyed widespread commercial success. It is believed that such systems fail to provide the needed flexibility, reliability and rapid response time to accommodate rough air, head and tail wind oscillations and other related problems.
It is now believed that there may be a commercial demand for an improved flare control system for landing an aircraft in accordance with the present invention. Such systems will provide a near asymptotic approach to touchdown and control the curvilinear flight path for the final 30 feet of descent. It is believed that there is a demand for a system and method which controls two parameters of importance, i.e. the pitch and thrust of the aircraft. Such systems must maintain certain minimum airspeed and at the same time provide sufficient lift to avoid hard landings and to prevent the aircraft from stalling.
In addition, the accuracy of the systems in accordance with the present invention provide enhanced landing performance on all landings including those made under adverse conditions. Adverse conditions include times when a runway is reflective, such as dark wet surfaces and patches of scud. Seaplane pilots are familiar with similar difficulties encountered in landing on glassy water and recognize that glassy water is one of the primary causes of landing accidents.
The systems in accordance with the present invention can be used on every landing with reliability. Further, such systems do not add any additional weight to an auto throttle computer and have been tested on the assignee""s aircraft regardless of whether and with good results. Also, the system provides auto throttle closure whenever the speed is above the altitude relationship or in case of head wind sheer. Further, the auto throttle closure will cease whenever the airplane is under the altitude/target speed or in the case of tail wind sheer and eliminates excessive floating before touchdown and hard landings.
In essence, the present invention contemplates a flare control system for providing a near asymptotic approach to touchdown by controlling the pitch and thrust of the aircraft at preselected altitudes above ground level. The system comprises a conventional flight director and conventional means for indicating the pitch angle of an aircraft during a landing maneuver and for displaying an indication of the pitch angle on a flight director. The system also includes a flare computer including a control law inputted into the computer. The control law defines a baseline pitch angle of the aircraft established in the last phases of flight which is modified as a function of height above ground level. A radio altimeter is operatively connected to the flare computer for producing a signal indicative of the height of the aircraft. Means including the flare computer generate a signal indicative of the pitch deviation from the control law to allow a pilot or auto-control to adjust the pitch to comply with the control law. Thrust control means for a pilot or auto control to retard the throttles as a function of altitude above ground level is also provided.
The computation inputs utilized by the present system include, glide path memory, airspeed, reference speed, radio altitude, rate of descent and air speed vs. altitude program.